Age and tectonic implications of Paleoproterozoic granitoid intrusions within the Nain Province near Nain, Labrador

Abstract

U-Pb (zircon) age determinations from three granitoid intrusions in the Nain region, formerly assigned to the well-known 1.35-1.29 Ga (Mesoproterozoic) Nain Plutonic Suite (NPS), indicate that they were emplaced, instead, during the Paleoproterozoic. Crystallization ages of 2109 ± 3 Ma for Sheet Hill granite, 2052 ± 4 Ma for Loon Island granite, and 2025 ± 7 Ma for Satok Island monzonite demonstrate a decreasing time of emplacement from north to south. Leucocratic basic rocks, superficially similar to those of the NPS, occur as inclusions within the Satok Island monzonite and are intruded by an 1873 ± 4 Ma granitic aplite dyke north of Webb's Bay. The ages clearly demonstrate granitic magmatism in the Nain area for at least 750 Ma, and anorthositic magmatism for at least 675 Ma, before the development of NPS. The earliest Paleoproterozoic magmatism (2109-2025 Ma) may have coincided with breakup of the North Atlantic craton, whereas the local magmatism represented by the 1873 Ma aplite dyke may be related to the embryonic stages of the continental collision that produced the 1860-1740 Ma Torngat Orogen. Leucocratic basic rocks associated with the Paleoproterozoic granitic rocks identified here could be Archean in age, but are more likely coeval with the granites. Other granitic and basic rocks in the Nain area may likewise be products of this bimodal plutonism. Identification of this possible geological duality suggests repetitive magmatism of "anorogenic type" over a significant time span in this part of Labrador. If the mantle-plume-related genesis accorded the Mesoproterozoic NPS is applicable to the Paleoproterozoic rocks, it implies multiple periods of similar lithosphere-asthenosphere interaction in this area, beginning at least in the Paleoproterozoic. The Paleoproterozoic magmas may have been emplaced into Nain Province crust along a linear intracontinental extension zone inboard from a rifted margin.